A typical single-junction photovoltaic cell is comprised of a substrate on which to form the device, two ohmic contacts to conduct current to an external electrical circuit, and two or more semiconductor layers in series to form the semiconductor junction. At least one of these semiconductor layers (the absorber) is chosen so that its bandgap is of a value for near-optimum conversion of solar radiation. In the typical design, one semiconductor layer is doped n-type, and the adjacent layer is doped p-type. The intimate proximity of these layers forms a semiconductor p-n junction. The p-n junction provides an electric field that facilitates charge separation in the absorber layer(s) when the cell is illuminated, and charge collection at the ohmic contacts.
In general, the typical photovoltaic cell comprises a substrate to mount the cell and two ohmic contacts to conduct current to an external electrical circuit. In this arrangement, the cell may also include two or more semiconductor layers in series.
In the standard photovoltaic cell including the substrate for mounting the cell and two ohmic contacts for conducting current to an external electrical circuit, in addition to the n-type layer and the p-type layer of a p-n junction cell, a three layer cell can include an intrinsic (i-type) semiconductor layer disposed between the n-type layer and the p-type layer for absorption of light.
In the photovoltaic cell, the semiconductor layers may be formed from single crystalline materials, amorphous materials, or polycrystalline materials. However, single crystalline materials are preferred from an efficiency perspective, because efficiencies are available in excess of about 20% in specific single crystalline photovoltaic cells Nevertheless, the disadvantages associated with single crystalline materials is the high cost of the material as well as the difficulty in depositing the single crystalline materials.
On the other hand, in the case of amorphous materials, one must contend with low carrier mobility, low minority carrier lifetime, low efficiency, and issues of cell stability. Therefore, while single-crystalline and amorphous materials are utilized in some photovoltaic device applications, semiconductor layers composed of polycrystalline materials are viewed as the preferred alternative for the production of photovoltaic devices that would be economically viable for a wide range of applications.
Polycrystalline materials offer numerous advantages for the production of photovoltaic cells. However, there is a desire in the industry of the field of polycrystalline materials to increase the efficiency of the polycrystalline photovoltaic cells from the current efficiencies of about 5-10% range to about a range of about 10-15%, and ultimately to advance the efficiencies of polycrystalline photovoltaic cells closer to the 15-25% range of single-crystalline materials.
Cadmium telluride is a semiconductor with electrical properties recognized in the industry as well suited for conversion of sunlight into electrical energy.The material has a bandgap that is nearly optimum for conversion of terrestrial solar radiation, and the ability to be doped n-type and p-type, that permits the formation of a large range of junction structures.
Nevertheless, there is still extant a need for more efficient and less expensive photovoltaic cells, and in particular, for a more efficient cadmium telluride photovoltaic cell suitable for large scale production. While much of the earlier stages of research on cadmium telluride solar cells was confined to the use of a single crystal cadmium telluride, more recent work has included processes for improving the deposition of cadmium telluride on a substrate. For instance, some examples of deposition processes now available include chemical vapor deposition, electrodeposition, close spaced sublimation, solid-gas reaction, sputtering spray pyrolysis, molecular beam epitaxy, liquid phase epitaxy, and other processes known in the art.
One significant technological problem with CdTe-based devices is that it is difficult to form an ohmic contact to the p-type form of the material. This is observed for both single crystalline and polycrystalline p-type CdTe, and results from a combination of large semiconductor work function, and the inability of CdTe to sustain sufficiently high p-type carrier concentration to enable quantum-mechanical tunneling of charge carriers at the CdTe/metal contact interface. In addition to these fundamental problems, the polycrystalline p-type CdTe material used as the absorber in a CdS/CdTe photovoltaic device is typically treated with Cl-containing liquids or vapors just prior to the formation of ohmic contact. The Cl treatments improve junction performance, but also can produce a CdTe surface that is rich in Cl. Furthermore, the formation of oxide layers from atmospheric oxygen or other processes can alter the chemical properties of the p-type CdTe surface. These factors can effect the electrical transport at the contact interface, and alter the characteristics of the ohmic contact.
The outer surfaces of the p-type CdTe are typically preconditioned prior to application of contact layers by the use of various surface treatments that are known in art. In addition to removing unwanted contamination from the surface, these preconditioning treatments can result in a CdTe surface that is relatively stoichiometric, or a CdTe surface comprised of a Te-rich outer layer. Regardless of the processes used to precondition the CdTe surface, the materials and processes that are subsequently used to produce the outer contact layers must be optimized for the characteristics of the preconditioned CdTe surface so that all interfaces produce low resistance and interface stability.
Typically, the processes used to deposit the outer metallization layers are performed at relatively low temperature to avoid diffusion of the outer contact layers into the CdTe (i.e., less than about 150.degree. C.). This low-temperature processing is undesirable for in-line photovoltaic cell manufacturing processes because higher-temperature processes (greater than about 150.degree. C.) typically occur immediately prior to the contact fabrication. Additionally, it is difficult to attain optimum adhesion of the contact metallization at low temperature, and this can result in reduced contact stability.
Accordingly, there is a need in the art of preparing contacts to the back surface of CdS/CdTe thin-film photovoltaic devices to produce improved contacting processes. Desirable improvements to the contacting process would include: Inherent compatibility with in-line manufacturing processes (i.e., use of "dry processes"); allowance for incorporation of dry processes to precondition the CdTe surface prior to contact fabrication; ability to fabricate the contact at high processing temperatures (&gt;150.degree. C.); ability to choose a variety of outer metallizations depending on industrial design considerations; ability to modify the contacting process to enhance contact stability; and provision to choose processes that minimize waste products.
The present invention encompasses the use of two separate ZnTe films deposited between the pre-conditioned p-CdTe surface and subsequent metallizations as interface layers to improve electrical contact to p-CdTe thin films. The first film deposited (on the p-CdTe) is undoped ZnTe, and the second film deposited is doped heavily p-type. The two separate films are used to avoid the drawbacks associated with using a single film to provide optimum device performance, and these layers may be used in photovoltaic and other devices to improve device performance by decreasing the losses associated with the back contact and by increasing back contact stability.
A heterojunction p-i-n photovoltaic cell having at least three different semiconductor layers formed of at least four different elements comprising a p-type wide band gap semiconductor layer, a high resistivity intrinsic semiconductor layer, used as an absorber of light radiation, and an n-type wide band gap semiconductor layer is disclosed in U.S. Pat. No. 4,710,589. The intrinsic layer is in electrically conductive contact on one side with the p-type layer and on an opposite side with the n-type layer. First and second ohmic contacts are in electrically conductive contact with the p-type layer and the n-type layer.
In U.S. Pat. No. 5,393,675, there is disclosed a process for dry fabrication of CdS/CdTe photovoltaic devices; however, this patent utilizes only a single ZnTe layer, discloses that the ZnTe functions only as the p-type layer of a p-i-n PV structure, and that the contact is provided by metal layers. Further, there is no indication or appreciation that the ZnTe can be doped, and that if a multilayer contact is used, both layers must be metals (e.g., Cu and Au).
Journal Paper, A. Mondal et al., Solar Energy Materials and Solar Cells 26, 181 (1992), discloses first use of electrochemistry to form ZnTe and ZnTe: Cu, and identifies the use of only a single-layer ZnTe: Cu interface.
The use of thin (1-5 nm) Cu layers to dope but not shunt the p-CdTe of a CdS/CdTe device is disclosed in U.S. Pat. No. 4,735,662, in which use of a barrier layer on top of a thin Cu layer is employed to afford selection of other conduction layers. In essence, this patent discloses the use of a metallic layer (.about.3 nm Cu) to provide the necessary p-type doping in the CdTe to produce a low resistance contact. On this layer is placed both a barrier layer and the thicker conduction layers.
U.S. Pat. No. 4,319,069 discloses chemical treatment of a p-CdTe surface prior to contacting to improve the contact characteristics. HNO.sub.3 (oxidizing acid) plus H.sub.3 PO.sub.4 (leveling agent) is employed to form the Te layer. In this patent, the use of .about.5% HNO.sub.3 +H.sub.3 PO.sub.4 chemical pretreatment is used to improve the contact characteristics, and is a subtractive process.
Chemical treatment of a p-CdTe surface prior to contacting to improve the contact characteristics, by use of an oxidizing acid plus a reducing agent (hydrazine or metal alkalide) is disclosed in U.S. Pat. No. 4,456,630. The use of the chemical pretreatment to improve contact characteristic is subtractive processing, and in such a process, generally, there is very limited control over characteristics such as removal rate, selective grain-boundary etching, and thickness of the resulting Te-layer formation.
In, Journal Paper A. Fahrenbruch., Solar Cells 21, 399 (1987), there is a review of technology used to contact polycrystaline p-CdTe up to approximately 1987; and, in Journal Paper, F. Debbagh et. al., Solar Energy Materials and Solar Cells 31, 1 (1993), there is a disclosure of interdiffusion of elemental Cu and Te layers on polycrystalline CdTe.
U.S. Pat. No. 4,568,792 discloses the production of CdS and CdTe ternaries (with Zn, Se, etc.) and makes photovoltaic devices that are better than the standard CdS/CdTe device.